1. Field of the Invention
The present invention relates to a method for controlling a machine tool that has a spindle, rotating at a variable rotation speed about a spindle axis, which carries a facing head that is fitted with a tool whose radial distance from the spindle axis is adjustable by means of a control system, the distance and/or the rotation speed being modified as a function of a machining operation to be performed with the tool.
2. Related Prior Art
Machine tools of this kind having facing heads are known from the related art.
"Facing heads" are understood to mean arrangements with which it is, for example, possible to face-turn large surfaces of workpieces. The particular feature of these facing heads is that a facing tool can be displaced radially with respect to the spindle axis, so that the turning diameter is variable. In the case of numerically controlled machine tools, a further numerical axis is obtained in this fashion.
The facing head is, in this context, joined immovably to the spindle so that it rotates along with the spindle. Depending on the machining operation to be performed, a different tool sits in the facing head and is driven at a specific spindle rotation speed, the radial distance of said tool from the spindle axis generally being changed during the machining operation so that the trajectory of the tool about the spindle axis changes accordingly.
Provided in the facing head for radial displacement of the tool are, for example, helical gear sets that are actuated through the spindle axis by means of a displacement motor different from the spindle drive system. In this context, the tool is as a rule received in a slide that is radially displaced via the helical gear set. A balance weight that is shifted in the correspondingly opposite direction is often provided in order to compensate for the resulting imbalance.
It has now been found that, because of the centrifugal forces present as well as other influences, the trajectory of the tool about the spindle axis deviates from the distance set by the control system, so that machining inaccuracies occur as a result.
In the related art, the dimensional deviations occurring in this fashion are compensated for by the fact that test pieces of the workpiece to be produced in production series with the machine tool are manufactured first, and the dimensional deviations are determined on them. The parameters set via the control system are then correspondingly corrected for the distances, whereupon a new test piece is machined. This procedure is performed iteratively until sufficient dimensional accuracy is achieved on the machined workpiece. The parameters obtained in this fashion are then used to produce the workpieces in production series.
A disadvantage of this method is that it is very time-consuming due to the iterative search for the correct parameters. A further disadvantage is the fact that this method must be performed afresh for each new workpiece geometry.